New Scientist

Sunday, May 19, 2002

Nicotine stops new brain cells forming
Nicotine can kill brain cells and stop new ones forming in the hippocampus, a brain region involved in memory, says a French team. The finding might explain the cognitive problems experienced by many heavy smokers during withdrawal, they say.
The team allowed rats to self-administer doses of nicotine daily for six weeks. At blood nicotine levels comparable to those found in smokers, they found the creation of new neurons in the dentate gyrus in the hippocampus was cut by up to 50 per cent. Cell death also increased.
The implications for smokers and for those using nicotine gum or patches to help them give up are unclear, says Pier Piazza of France's National Institute for Health and Medical Research (INSERM) in Bordeaux, who was involved in the research.
He thinks the loss of "neuronal plasticity" could cause cognitive problems, but the team did not test this in the rats. "That is the next step. But the hippocampus is involved in memory and neurogenesis seems to be involved in memory, so we might expect there would be an effect," he told New Scientist.
Masking effect
Short-term cognitive problems in smokers trying to kick the habit are well documented. "But there has been no good explanation for this," Piazza says. "It could be that while they are smoking, the stimulant effect of nicotine masks the loss in neuronal plasticity. When they stop smoking, these deficiencies remain."
But Sue Wonnacott, an expert on nicotine at the University of Bath, UK, says: "There is a lot of redundancy in the brain and such deficits may or may not be sufficient to account for memory impairments. An alternative view is that memory impairments result from the subject feeling dreadful in the withdrawn state."
Exactly how nicotine kills cells is not clear. Previous research on fetal exposure suggest it can induce apoptosis - programmed cell death - in immature cells. Piazza says it is possible that nicotine can kill cells elsewhere in the brain. But the dentate gyrus is one of only two regions known to form new - and so immature - neurons in the adult brain.
Previous research has suggested that smoking might actually be beneficial for patients with a variety of diseases associated with brain cell loss. This is thought to be because nicotine can attach to receptors for acetylcholine, a neurotransmitter involved in memory.
"I don't want to argue with findings that acetylcholinergic agonists - like nicotine - can be beneficial for Alzheimer's," says Piazza. "But now we have found that nicotine can also kill brain cells."
Journal reference: Journal of Neuroscience (vol 22, p 3656)